Besides driving a powertrain, an engine of a vehicle provides power to various auxiliary components. These components typically include an alternator that recharges a battery, an A/C compressor for an A/C system, and/or a hydraulic pump that provides pressurized hydraulic fluid. Powering each of these auxiliary components reduces the torque output of the engine. During idle, the reduced torque output may cause noticeable fluctuation of engine idle speed.
Traditionally, controllers use spark retard and idle air control to reduce engine idle speed fluctuations. Both methods, however, have disadvantages. Spark retard causes inefficient engine operation during idle. Idle air control enables only gross tuning of the engine idle speed.
Some of the auxiliary components such as the alternator and A/C compressor provide feedback signals to an engine controller, which compensates for these loads. Other auxiliary components such as the hydraulic pump do not provide feedback signals to the engine controller. When the transmission load requires increased hydraulic pressure, the hydraulic pump increases the load on the engine, which fluctuates engine idle speed. Compensation does not occur until some time after the fluctuation occurs.